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Chapter 8Recovering Graphics Files
Guide to Computer Forensics and Investigations
Fifth Edition
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 2
Objectives
• Describe types of graphics file formats
• Explain types of data compression
• Explain how to locate and recover graphics files
• Describe how to identify unknown file formats
• Explain copyright issues with graphics
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 3
Recognizing a Graphics File
• Graphic files contain digital photographs, line art, three-dimensional images, and scanned replicas of printed pictures – Bitmap images: collection of dots
– Vector graphics: based on mathematical instructions
– Metafile graphics: combination of bitmap and vector
• Types of programs– Graphics editors
– Image viewers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 4
Understanding Bitmap and Raster Images
• Bitmap images– Grids of individual pixels
• Raster images - also collections of pixels– Pixels are stored in rows– Better for printing
• Image quality– Screen resolution - determines amount of detail– Software contributes to image quality (drivers)– Number of color bits used per pixel
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 5
Understanding Vector Graphics
• Characteristics of vector graphics– Uses lines instead of dots– Store only the calculations for drawing lines and
shapes– Smaller than bitmap files– Preserve quality when image is enlarged
• CorelDraw, Adobe Illustrator
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 6
Understanding Metafile Graphics
• Metafile graphics combine raster and vector graphics
• Example– Scanned photo (bitmap) with text (vector)
• Share advantages and disadvantages of both types– When enlarged, bitmap part loses quality
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 7
Understanding Graphics File Formats
• Standard bitmap file formats– Portable Network Graphic (.png)– Graphic Interchange Format (.gif)– Joint Photographic Experts Group (.jpeg, .jpg)– Tagged Image File Format (.tiff, .tif)– Window Bitmap (.bmp)
• Standard vector file formats– Hewlett Packard Graphics Language (.hpgl)– Autocad (.dxf)
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 8
Understanding Graphics File Formats
• Nonstandard graphics file formats– Targa (.tga)– Raster Transfer Language (.rtl)– Adobe Photoshop (.psd) and Illustrator (.ai)– Freehand (.fh9)– Scalable Vector Graphics (.svg)– Paintbrush (.pcx)
• Search the Web for software to manipulate unknown image formats
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 9
Understanding Digital Camera File Formats
• Witnesses or suspects can create their own digital photos
• Examining the raw file format– Raw file format
• Referred to as a digital negative
• Typically found on many higher-end digital cameras
– Sensors in the digital camera simply record pixels on the camera’s memory card
– Raw format maintains the best picture quality
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 10
Understanding Digital Camera File Formats
• Examining the raw file format (cont’d)– The biggest disadvantage is that it’s proprietary
• And not all image viewers can display these formats
– The process of converting raw picture data to another format is referred to as demosaicing
• Examining the Exchangeable Image File format– Exchangeable Image File (Exif) format
• Commonly used to store digital pictures
• Developed by JEITA as a standard for storing metadata in JPEG and TIF files
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 11
Understanding Digital Camera File Formats
• Examining the Exchangeable Image File format (cont’d)– Exif format collects metadata
• Investigators can learn more about the type of digital camera and the environment in which pictures were taken
– Viewing an Exif JPEG file’s metadata requires special programs
• Exif Reader, IrfanView, or ProDiscover
– Exif file stores metadata at the beginning of the file
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 12
Understanding Digital Camera File Formats
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 13
Understanding Digital Camera File Formats
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 14
Understanding Digital Camera File Formats
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 15
Understanding Digital Camera File Formats
• Examining the Exchangeable Image File format (cont’d)– With tools such as ProDiscover and Exif Reader
• You can extract metadata as evidence for your case
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 16
Understanding Digital Camera File Formats
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 17
Understanding Data Compression
• Some image formats compress their data– GIF and JPEG
• Others, like BMP, do not compress their data– Use data compression tools for those formats
• Data compression– Coding data from a larger to a smaller form– Types
• Lossless compression and lossy compression
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 18
Lossless and Lossy Compression
• Lossless compression– Reduces file size without removing data– Based on Huffman or Lempel-Ziv-Welch coding
• For redundant bits of data
– Utilities: WinZip, PKZip, StuffIt, and FreeZip• Lossy compression
– Permanently discards bits of information– Vector quantization (VQ)
• Determines what data to discard based on vectors in the graphics file
– Utility: Lzip
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 19
Locating and Recovering Graphics Files
• Operating system tools– Time consuming– Results are difficult to verify
• Digital forensics tools– Image headers
• Compare them with good header samples
• Use header information to create a baseline analysis
– Reconstruct fragmented image files• Identify data patterns and modified headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 20
Identifying Graphics File Fragments
• Carving or salvaging– Recovering any type of file fragments
• Digital forensics tools– Can carve from file slack and free space– Help identify image files fragments and put them
together
© Cengage Learning 2015
Repairing Damaged Headers
• When examining recovered fragments from files in slack or free space– You might find data that appears to be a header
• If header data is partially overwritten, you must reconstruct the header to make it readable– By comparing the hexadecimal values of known
graphics file formats with the pattern of the file header you found
Guide to Computer Forensics and Investigations, Fifth Edition 21
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 22
Repairing Damaged Headers
• Each graphics file has a unique header value
• Example:– A JPEG file has the hexadecimal header value
FFD8, followed by the label JFIF for a standard JPEG or Exif file at offset 6
• Exercise:– Investigate a possible intellectual property theft by a
contract employee of Exotic Mountain Tour Service (EMTS)
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 23
Repairing Damaged Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 24
Repairing Damaged Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 25
Searching For and Carving Data from Unallocated Space
• Steps– Planning your examination– Searching for and recovering digital photograph
evidence• Use ProDiscover to search for and extract (recover)
possible evidence of JPEG files
• False hits are referred to as false positives
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 26
Searching For and Carving Data from Unallocated Space
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 27
Searching For and Carving Data from Unallocated Space
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 28
Searching For and Carving Data from Unallocated Space
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 29
Searching for and Carving Data from Unallocated Space
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 30
Searching for and Carving Data from Unallocated Space
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 31
Rebuilding File Headers
• Before attempting to edit a recovered graphics file– Try to open the file with an image viewer first
• If the image isn’t displayed, you have to inspect and correct the header values manually
• Steps– Recover more pieces of file if needed– Examine file header
• Compare with a good header sample
• Manually insert correct hexadecimal values
– Test corrected file
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 32
Rebuilding File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 33
Rebuilding File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 34
Rebuilding File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 35
Rebuilding File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 36
Rebuilding File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 37
Reconstructing File Fragments
• Locate the noncontiguous clusters that make up a deleted file
• Steps– Locate and export all clusters of the fragmented file– Determine the starting and ending cluster numbers
for each fragmented group of clusters– Copy each fragmented group of clusters in their
correct sequence to a recovery file– Rebuild the file’s header to make it readable in a
graphics viewer
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 38
Reconstructing File Fragments
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 39
Reconstructing File Fragments
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 40
Reconstructing File Fragments
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 41
Reconstructing File Fragments
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 42
Identifying Unknown File Formats
• Knowing the purpose of each format and how it stores data is part of the investigation process
• The Internet is the best source– Search engines like Google– Find explanations and viewers
• Popular Web sites– www.fileformat.info/format/all.htm– http://extension.informer.com– www.martinreddy.net/gfxl
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 43
Analyzing Graphics File Headers
• Necessary when you find files your tools do not recognize
• Use a hexadecimal editor such as WinHex– Record hexadecimal values in the header and use
them to define a file type
• Example:– XIF file format is old, little information is available– The first 3 bytes of an XIF file are the same as a TIF
file – Build your own header search string
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 44
Analyzing Graphics File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 45
Analyzing Graphics File Headers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 46
Tools for Viewing Images
• After recovering a graphics file– Use an image viewer to open and view it
• No one viewer program can read every file format– Having many different viewer programs is best
• Most GUI forensics tools include image viewers that display common image formats
• Be sure to analyze, identify, and inspect every unknown file on a drive
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 47
Understanding Steganography in Graphics Files
• Steganography hides information inside image files– An ancient technique
• Two major forms: insertion and substitution
• Insertion– Hidden data is not displayed when viewing host file
in its associated program• You need to analyze the data structure carefully
– Example: Web page
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 48
Understanding Steganography in Graphics Files
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 49
Understanding Steganography in Graphics Files
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 50
Understanding Steganography in Graphics Files
• Substitution– Replaces bits of the host file with other bits of data– Usually change the last two LSBs (least significant bit)– Detected with steganalysis tools (a.k.a - steg tools)
• You should inspect all files for evidence of steganography
• Clues to look for:– Duplicate files with different hash values– Steganography programs installed on suspect’s drive
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 51
Understanding Steganography in Graphics Files
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 52
Understanding Steganography in Graphics Files
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 53
Understanding Steganography in Graphics Files
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 54
Using Steganalysis Tools
• Use steg tools to detect, decode, and record hidden data
• Detect variations of the graphic image– When applied correctly you cannot detect hidden
data in most cases
• Check to see whether the file size, image quality, or file extensions have changed
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 55
Understanding Copyright Issues with Graphics
• Steganography has been used to protect copyrighted material– By inserting digital watermarks into a file
• Digital investigators need to aware of copyright laws
• Copyright laws for Internet are not clear– There is no international copyright law
• Check www.copyright.gov– U.S. Copyright Office identifies what can and can’t
be covered under copyright law in U.S.
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 56
Summary
• Three types of graphics files– Bitmap– Vector– Metafile
• Image quality depends on various factors• Standard file formats: .gif, .jpeg, .bmp, and .tif• Nonstandard file formats: .tga, .rtl, .psd, and .svg• Some image formats compress their data
– Lossless compression– Lossy compression
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 57
Summary
• Digital camera photos are typically in raw and EXIF JPEG formats
• Recovering image files– Carving file fragments– Rebuilding image headers
• The Internet is best for learning more about file formats and their extensions
• Software– Image editors– Image viewers
© Cengage Learning 2015Guide to Computer Forensics and Investigations, Fifth Edition 58
Summary
• Steganography– Hides information inside image files– Forms
• Insertion
• Substitution
• Steganalysis– Finds whether image files hide information